This invention relates to the prestructuring of thermomagnetic imaging recording members by the impression of patterns of magnetic gradients and, more particularly, to the orientation of such patterns of magnetic gradients.
Reduced to its fundamental elements, thermomagnetography employs a recording member on which to record the image of a document. Recording of the image is obtained by the selective, imagewise demagnetization of portions of the recording document through exposure to radiation which selectively, in imagewise form, raises momentarily the temperature of portions of the recording member to above its Curie temperature. The latent image thus produced is made visible by the application of a magnetic toner which is attracted to the magnetized portions of the recording member. A stream of air may be used to help completely remove toner from the unmagnetized portions and provide a cleaner background. The process may also involve transferring this toner from the recording member to a receiving member and fixing it on the receiving member to provide a permanent record.
Magnetography in general and useful materials for its implementation are described in U.S. Pat. No. 3,555,556 (NACCI).
The prestructuring of the magnetic member through the imposition of a pattern of magnetic gradients is quite important in obtaining a high quality image in thermomagnetography, as it controls to a great extent the toner attraction to the nondemagnetized portions of the magnetic member surface to form the image. In U.S. Pat. No. 3,781,903 (Jeffers et al.) is disclosed a correlation between the frequency of the magnetic gradient pattern and the size of the magnetic toner to optimize resolution and contrast aspects of a thermomagnetically produced image.
Image quality tends to deteriorate as one attempts to reproduce finer and finer lines. This can be understood if one considers that the magnetic image has been created by the selective demagnetization of a magnetic member which has a preimposed magnetic structure in its surface. This magnetic structure may be represented as a pattern of parallel closely spaced lines which present maximum toner attraction separated by spaces of no toner attraction. As the frequency of this premagnetization is optimized, toner covers uniformly the magnetic member. When very fine image lines are produced through demagnetization of the prestructured member, magnetic bridging occurs across the small gaps and it is difficult to prevent toner particles from adhering to the demagnetized areas. Because image lines are rarely parallel to the preimposed magnetic lines, this effect appears as a moire pattern on portions of the image. It is desirable, therefore, to provide a method for the elimination of this problem, preferably through improved cleanability of the toner from the background.
It is an object of this invention to provide better cleaning of the background areas of the toned thermomagnetic member. It is a further object of this invention to sufficiently clean the background areas of the toned thermomagnetic member, especially when fine line patterns are present, to minimize the presence of moire patterns in the toned image.